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Home – Techniek – Electronica – Radiotechniek – Radio amateur bladen – Radio Communication – A power supply and control system for tetrode amplifiers 2. Refer now to Fig 2, which is a dual-channel screen supply capable of producing a regulated or V for Class AB I and Class C use respectively and which will source and sink some 30mA; it is a development of the design published in III. The description refers to one channel, which is Channel B the right-hand channel looking at the circuit diagram.
BU (Mospec) – POWER TRANSISTORS(A,V,55W)
The raw input – which should be V or above – is fed via the optoisolator IC4b, whose function was discussed earlier, to the collector of TR8. With associated components this forms a constant-current source. The op-amp IC10 has its inverting input voltage derived from the 82V reference from D39, which is compared with a sample voltage derived from the output rail in the resistor network R63, R64 and R This sample of the output voltage is datasjeet with the reference voltage from D39 in ICIO, which drives the shunt stabiliser element TR9.
The output voltage is thus kept constant. Those familiar with the will note that in this circuit the compensation capacitor has a considerably greater value than is usual. This “overcompensation” technique can be useful for ensuring stability in regulator circuits where the total open-loop gain is extremely high, as it is in this design. Note that an internally-compensated op-amp such as the cannot be used in this circuit. The supply voltage for the op-amps is produced in a slightly unorthodox manner dxtasheet D45 and D46 and the associated capacitors; these form a voltage doubler for which the zener diodes provide protection against stray transient voltages across the rails and enable the screen supply op-amps to be powered from a spare 6.
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The output voltage of the B channel is fixed; however, the output voltage of the A channel can be varied by RV14 and RV The intention here is to enable the standing currents in a two-valve amplifier to be equalised, which is better done by variation of screen voltage than of control grid bias to individual valves. In fact they are something of a backstop since GW4FRX’s amplifers have for some years had a similar component directly between pin 1 of the valveholder and chassis and this seems to have worked well in protecting the valve and its base against flashover damage.
The performance of this design is very good. If 30mA or less is being sourced or sunk, the consequent variation in output voltage is not measurable on a 4. However, if more than some 40mA is drawn from the supply the output voltage quickly falls to a safe value so that the screen dissipation of the valve cannot be exceeded even under fault conditions. As would be expected of a shunt stabiliser, it has no objection to being short-circuited. For the 4CX and family in linear service with about 2kV on the anodes and V on the screens, the optimum standing current for best third- and fifth-order intermodulation performance seems to be about mA per valve, as the data sheet suggests.
Fig 5 shows a suggested scheme. This diagram also shows how the screen feed arrangements of many amplifier designs might profitably be modified to minimise both the risk of flashover and the consequent damage. The two 68kf12W resistors and the vdr should be connected directly to pin 1 of the valveholder.
Allowance will obviously have to be made in the screen metering for the 10mA drawn by the resistors, but this can be turned to advantage: Positive or negative screen current can then be read without the necessity for switching. The inductor is formed by winding about 10 turns of 22swg enamelled wire around the body of a low-value 2W resistor. In conjunction with the built-in capacitor in the valve base and the pF capacitor shown on the diagram, this forms an rf lowpass filter. Most 4CX amplifer designs include a low-value resistor in series with the screen feed to each valve, but it is difficult to see what this is intended to achieve.
It can have little or no anti-parasitic effect, and in the event of a flashover the resistor will certainly force the voltage on the screen of the valve to rise to a very high value, which will almost invariably destroy the capacitor in the valve base.
In general terms, the best thing to do with this resistor is to wind an inductor round it.
Suggested screen grid circuitry. In a series of some 20 test flashovers, which were induced by tuning and loading a W1SL-type MHz amplifier with these modifications incorporated for full output power into a dummy load, removing the load and applying full drive, there was no damage to any part of the amplifier or its power supply apart from blown fuses in the ehv supply and normal operation could be resumed as soon as the fuses had been replaced and the load had been reconnected.
Incidentally, and contrary to the recommendations of some writers on the subject, fuses should not be inserted in series with the screen feed lines.
The authors datwsheet of no fuse which will fail at a guaranteed 30mA or so with any consistency, and very-low-current fuses are prone to early fatigue failures with varying current.
With a correctly-designed screen supply ;here is no need for fuses, and they can be positively dangerous if fuse failVre leaves the screen grid floating; flashover and destruction of the valve-base is likely to result. Neither the control grid nor the screen grid of valves in the 4CX datashet should ever be allowed to float with no reference to cathode if damage is to be avoided.
The changeover relay earths the screen on receive and connects it to the output of the screen suRply on transmit. The bleeder resistors and vdr are connected directly to the.
Earthing the screen grid during receive periods avoids shot noise in the receiver, and also any chance of the capacitor in the valve base charging as a result of dstasheet emission effects and taking the screen voltage to a high value. A further advantage of the changeover system is that any alarm in the control unit or failure of the bu2008 can be made to earth the screen grids, using the “ready” relay RLD as discussed above.
Front view of the complete equipment. There are various devices which can be used in this design. The cheapest option is to use BU television line output transistors for TR and these will work well even though the hk of some BUs has been found to be very low.
It should be added, however, that under short-circuit conditions a BU used in the constant-current source ie TR6 and TR8 is taken some way outside its published safe operating area S.
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R and should therefore be theoretically destroyed when the supply is short-circuited because of the tiresome bipolar transistor affliction known as “second breakdown”. The authors would merely say that in some five years of using both datsheet design and its predecessor, which has been built by many constructors, no cases of device failure under short-circuit conditions have been experienced or reported to them.
However, purists may wish instead to use one of the family of transistors developed in recent years for high-voltage switched-mode power supply use, whose S. If particularly esoteric devices with very high hk are used it may datashewt necessary to increase the values of R50 and R52, which set the short-circuit current. The quoted value gives about 60mA with most of the above transistors and input voltages between and V.
As far as the shunt stabiliser elements TR7 and 9 are concerned, all the above devices have been tried and work well. The BUT16 is bk208 1. The authors suspect that several other tv line output or smps transistors could be used in this part of the circuit, although they have not been tried. However, high-voltage Darlingtons will not work efficiently in the constant-current source position. Another possibility for the shunt stabiliser and indeed for the constant-current source is a power mosfet.
Any device with a Vdss rating in excess of about V and a current rating of an amp or so should work well, although minor component changes need to be made. If a mosfet is to be used in the constant-current source, D32 and D33 should be changed from 6V8 datashheet 8V2 devices and the value of R50 and R52 should be increased to 68k 2W. For use in the sbunt stabiliser element, C32 and C37 should become 1nF and a 2.
The diodes on the output of each op-amp, D34 and D44, should be removed and links fitted on the pcb in their place. Tests on one of the prototypes were carried out using IRF devices and very good results were obtained; second breakdown and S.
The authors have not tried a mosfet-fitted version “on-air”, as it were, but there is no reason why it should not work perfectly well. Finally, TR10 and TR11 can be almost any small-signal npn device; since the BC seems to be in many people’s junk boxes, that is the one shown on the circuit diagram.
The zener diode D35 is a 10W stud-mounting component. It is worth noting that devices in the BZY93 series are available with either the cathode or the anode as stud, and in the present design the stud anode variant denoted by the “R” suffix following the voltage in the type number is obviously more convenient.
Fig 3 is the circuit of the bias supply. The heart of it is an overcompensated driving a shunt stabiliser transistor, TR5. Operating voltage is derived from a three-terminal regulator IC7, which can also be used to produce a variable negative-going alc voltage for power control of many commercial transceivers. The first is to fit D48 and R68 and to omit link 1; this option is used with the circuit shown in Fig dxtasheet.
The value of R68 should be chosen to pass about 5mA when the ptt switch in Fig 6 is open-circuit; the formula is. On transmit, C26 in the bias supply delays turn-on of the valve s until both antenna and screen changeover relays have operated. When switching to receive, the back emf from the relays causes the voltage at R68 to rise datasheeet quickly; the output from the bias supply therefore goes negative and takes the valve s out of conduction before the relays actually open.
HIGH VOLTAGE TRANSISTORS BU208
The result is that the antenna changeover relays never switch rf and there are no heavy pulses of datasheef current at the changeover. If D48 and R68 are omitted and link 1 fitted, leaving pin 72 open-circuit corresponds to the transmit condition. Extending an earth to pin 72 switches the supply to the receive mode. This option may be more suitable for some users. The performance of this supply is also good. Because its reference is a 5V1 zener diode-a value which has a temperature coefficient of virtually zero-the long-term stability of the output voltage is excellent.
Since the catasheet will sink up to 60mA without difficulty, there are no problems of fluctuation of the working point of the valve datasheet a function of grid current.
The output ripple of the bias supply is less than 2mV pk-pk. The range of transistors which can be used for TR5 is a little limited.
Completed pcb mounted in die-cast box. Beginning with the screen supply, it will be noted that there arc two removable links between the constant-current sources and the shunt stabilisers. These are for initial setting-up of the standing currents, as discussed later. The links can either be made from Veropins joined with wire or the small “U-links” plugging into miniature pcb sockets which are available from Maplin or Electromail.
RV 12 and RV 13 are ordinary pcb-mounting cermet potentiometers; all xatasheet variable resistors on the pcb were of the small multiturn wirewound variety in the prototypes, although provision has been made on the board for other styles of component and some economies can be made here if required.
R52 and R50 need to have a rating of at least 2W and can either be metal-oxide resistors or chassis-mounting wirewound components bolted to the side of the die-cast box as shown in the photograph. The constant-current source and shunt stabiliser transistors require to be mounted on a heatsink, although they are dissipating relatively catasheet power and the chassis could serve the purpose.
The wiring between the transistors and the pcb should be kept reasonably short and direct, and screened multiway cable should be used. Although the D-type connector is usually associated with the strange world of RS and low digital voltages, the voltage ratihg of most D-type connectors is V dc or peak ac, and there have been no difficulties in using them to connect the screen supply transistors to the parent circuitry.
It will be noted that in the photograph of the completed unit mounted in its die-cast box there is an additional small pcb to the left of the main board. In the unit built at GW4FRX it was decided to save front-panel space by combining the screen over and under-voltage alarm l. Unfortunately it was discovered that all bicolour l. The four extra transistors on the additional board are simple inverters to interface the cmos with common-cathode bicolour l.
Note that the main rectifiers for both the screen supply and bias supply are both mounted on the pcb and, in the case of the latter, so is its reservoir capacitor.
Coming now to the bias supply, R43 is also a 2W component and is subject to the same comments as R52 and R50 above. Other devices may require some minor rearrangement of their mounting to suit the pcb pinouts.
RLA-D are small pcb-mounting two-pole changeover relays which are available from various sources: As far as the control logic is concerned, RR38 are all 2. The positive regulator, IC5, can be mounted on the die-cast box for heatsinking purposes, although the negative regulator 1C7 does not require a heatsink.
The main timing capacitor, C15, should be a tantalum component; C14 should ideally be tantalum also, although in practice a conventional electrolytic will work.